Recently active-matrix organic light-emitting diode (OLED) displays have become more attractive due to advantages over conventional liquid crystal flat displays. These advantages include the ability to fabricate OLED displays at a relatively low cost and high efficiency. Further the displays do not require backlighting and provide a wide viewing angle.
An active-matrix organic light-emitting diode (AMOLED) display compromises an array of rows and columns of pixels, each having an OLED and some active devices such as thin film transistors. Since OLEDs are current driven devices the pixel circuit of an AMOLED should be capable of providing an accurate and constant drive current to achieve a consistent and uniform luminance.
As disclosed in U.S. Pat. NO. 5,748,160, a simple pixel circuit comprises two thin film transistors (TFTs) and an OLED. In this circuit, the OLED is connected to the drain terminal of a driving TFT and a gate terminal of the driving TFT is connected to a column line through a switching TFT. A storage capacitor connected between the gate terminal of the driving TFT and ground is used to maintain the voltage at the gate terminal of the driving TFT when the pixel circuit is disconnected from column line. For this circuit the current through OLED strongly depends on the characteristic parameters of the driving TFT. Since the characteristic parameters of a TFT, particularly, the threshold voltage under bias stress, vary by time, and such changes may differ from pixel to pixel, the induced image distortion may be unacceptably high.
One of the methods that has been employed to make the current driving circuit less sensitive to the shift in the threshold voltage is programming the pixel with current instead of voltage. In this method, the OLED current is less dependent on the voltage-current characteristics of driving transistors. Implementations of current programmed pixel circuits for OLEDs have been disclosed e.g. Yi HE et al., “Current Source a-Si:H Thin-Film Transistor Circuit for Active Matrix Organic Light-Emitting Displays”, IEEE Electro Device Letters, Vol. 21, No. 12, p 590-592, December 2000).
A drawback of the current programming method is that it is slow, particularly for low programming current levels, due to the large line capacitance. As a result, voltage programming methods are desirable considering their speed. This is particularly true for large area TVs and displays.
Another method to make the drive current less sensitive to transistor parameters is to use current feedback. United States patent application 20020101172A1 provides a driving system with current feedback. An external current comparator compares the pixel current with a reference current and generates an appropriate signal to control the pixel current. One drawback of the disclosed method is that the control signal is a current, which can limit the programming speed. Another drawback of the method is that the anode and cathode electrode of each OLED have to be patterned, which creates reliability concerns in the currently used OLED fabrication process.
Luminance feedback is another method that has been used to stabilize OLED luminance. As described in U.S. patent application 20030151569 feedback readout circuits responsive to the feedback signal representing the light output of the OLED can be used to provide brightness control. A drawback of the disclosed method is that every pixel requires a photo-sensor that is optically coupled to the OLED. This results in integration issues. Another drawback is that the low level of the feedback signal generated by a photo-sensor may lead to the poor signal-noise ratio, thereby narrowing the dynamic range of the system.